Fine structure and cytochemistry of the oocyte nucleolus in the mollusk Helcion pellucidus (Prosobranchia)

Abstract

Several nonchromatin masses occur at the periphery of the nucleus in early prophase meiotic oocytes, gain an electronlucent core, and at diplotene give rise to a large single nucleolus. The light homogeneous core is strongly stained at the light microscope level with eosin, amido black 10B, and fast green FCF, and it takes up tritiated amino acids but no [ 3 H]uridine. The peripheral dense material consists of separate fibrillar and granular areas. The fibrillar component is stained at electron microscope level with the Ag·NOR method and takes up [ 3 H]uridine at pulse incubation time, while the granular component is labeled at chase times. At dictyate, the nucleolus becomes very large (∼9800 μ m 3 ), the core mass gradually disappears leaving a vacuolar space while small extra-nucleolar bodies appear which have the same cytochemical behavior as the core; the fibrillar component continues to be silver stained and strongly labeled with [ 3 H]uridine. After middictyate there is extrusion of the granular component to the nucleoplasm and a gradual decrease and final disappearance of the nucleolus. The occurrence of a fast growing and actively synthesizing enormous nucleolus suggests massive ribosome production in these oocytes despite the lack of nucleolar multiplication. The existence of an associated proteinaceous mass may be interpreted as a source of ribosomal protein or of yolk material, although it may also consist of skeletal protein.